1. Field of the Invention
The present invention relates to a power conversion apparatus such as a frequency converter or a rectifier and, more particularly to such an apparatus equipped with harmonic filtration for eliminating inexpedient higher harmonics.
2. Description of the Background Art
Conventionally, frequency conversions of power supplies have been performed by devices such as a thyristor inverter or a cycloconverter. On the other hand, rectifications have been performed by devices such as a thyristor Leonard equipment.
These devices have in common a property that their switching frequencies are relatively low. This property causes the appearance of higher harmonics besides the fundamental one in their output currents.
These are devices whose outputs contain lesser amount of higher harmonics such as those using a power transistor or a gate turnoff. However, they are expensive to manufacture and have low conversion efficiency so that they are not suitable for dealing with large quantities. Furthermore, these devices induces higher harmonics in the power supply which perturb supply voltage, and this may affect the other apparatuses sharing the power supply.
On the other hand, there is a constraint regarding higher harmonics arising from the loading process. Namely, there is a case where a shaft connecting a motor and a driver is long and its resonant frequency coincides with that of a higher harmonic. An instance of such a case is when a power converter is used for variable-speed control of an alternating current motor where the presence of higher harmonics causes the appearance of torque-ripple, and the problem of reasonant vibration of a shaft arises. Conventionally, such a case has been treated either by suppressing operations at the resonant frequency, or by employing devices with a lesser amount of higher harmonics of DC reactor, all having a common drawback of low conversion efficiency.